3,3,4,4-tetrachlorotetrahydrothiophene-1,1-dioxide has the structural formula: ##SPC1##
This compound, its preparation and its utility as a biologically active chemical are described in U.S. Pat. No. 2,957,887, issued Oct. 25, 1960, in the names of Reynold A. Berkey and Henry Bluestone. As conventionally practiced heretofore on a commercial scale, preparation of the compound has proceeded via a stepwise chlorination process, starting with butadiene sulfone, a cyclic unsaturated butadiene derivative. Butadiene sulfone is also designated chemically as 2,5-dihydrothiophene-1,1-dioxide. Initially, the cyclic starting material in solution in an inert solvent such as carbon tetrachloride is saturated by chlorination across the double bond, said addition chlorination being conducted at moderate temperatures. The resulting saturated 3,4-dichlorotetrahydrothiophene-1,1-dioxide product, likewise in solution, is chlorinated under moderate reaction pressure via photochlorination. That is to say, actinic light radiation is supplied to the reaction zone to serve as the chlorination catalyst.
However, the second-stage photochlorination step affords many disadvantages. In order to effect substitution chlorination to obtain 3,3,4,4-tetrachlorotetrahydrothiophene-1,1-dioxide in commercially feasible quantities, the sluggish, light-catalyzed chlorination reaction must usually be conducted for a time period of 50-60 hours. It must further be maintained under somewhat critical reaction conditions so that degradation of the product will be minimized during the long reaction time. Even so, degradation typically occurs with formation of polymeric derivatives as well as both trichloro- and dichlorotetrahydrothiophene-1,1-dioxide analogs which chlorinate to the pentachloro and hexachloro compounds. Thus, yields of the desired 3,3,4,4-tetrachlorotetrahydrothiophene-1,1-dioxide product oftentimes are less than commercially feasible. Chlorine reaction efficiencies for the overall chlorination reaction are poor, making byproduct hydrochloric acid disposal and/or extensive chlorine recovery equipment necessary in commercial operations. There has long been a need for a faster, more economical and commercially attractive method for converting butadiene sulfone to 3,3,4,4-tetrachlorotetrahydrothiophene-1,1-dioxide.